Unless otherwise stated, all the values relating to the chemical composition of the alloys are expressed as a percentage by weight.
The alloys usually used for the cylinder heads of mass-produced motor vehicles are on the one hand alloys of the type AlSi7Mg and AlSi10Mg, possibly “doped” by the addition of 0.50% to 1%, of copper, and on the other hand alloys of the family AlSi5 to AlSi5-9Cu3Mg.
The alloys of the first type, AlSi7(Cu)Mg and AlSi10(Cu)Mg with T5 treatment (simple stabilization) and T7 treatment (complete solution heat-treatment, quenching and over-ageing) have sufficient mechanical characteristics when hot up to approximately 250° C., but not at 300° C., a temperature which will nevertheless be reached by the valve bridges of the new generations of supercharged diesel engines with a common rail, and even the new doubly supercharged petrol engines.
At 300° C., their yield strength and their creep strength are particularly low. On the other hand, because of their good ductility throughout the temperature range, from ambient up to 250° C., they satisfactorily withstand cracking by thermal fatigue.
Alloys of the type AlSi5 to AlSi5-9Cu3Mg0.25 to 0.5, which have better elevated temperature strength, have, in contrast, rather low ductility which makes them very vulnerable to cracking by thermal fatigue.
They are subdivided into a family of alloys with low iron content, typically lower than 0.20%, known as primary alloys (obtained from a smelter), which has good hot ductility but remains fragile at ambient temperature, and a family of alloys known as secondary alloys (obtained from recycling) with a higher iron content, from 0.40% to 0.80% and sometimes 1%, which have low ductility both when hot and at ambient temperature.
These problems were described for example in the article by R. Chuimert and M. Garat “Choice of aluminum casting alloys for diesel cylinder heads subjected to strong forces” published in the SIA Review of March 1990. This article summarized the properties of the three alloys examined as follows:
AlSi5Cu3Mg with low iron content (0.15%) and in state T7: very good mechanical resistance up to 250° C., becoming average at 300° C., low ductility at ambient temperature, becoming good at 250 and 300° C.
AlSi5Cu3Mg with high iron content (0.7%) and in state F (without heat treatment): average mechanical resistance at ambient temperature, becoming relatively highest at 250 and 300° C., very low ductility throughout the field 20-300° C.
AlSi7Mg0.3 without copper and with low iron content (0.15%) and in state T7: mechanical resistance at ambient temperature good, becoming very low as of 250° C., very good ductility throughout the field 20-300° C.
The progress made since 1990 was described in the recent article by M. Garat and G. Laslaz “Improved aluminum alloys for diesel cylinder heads” published in the review “Hommes et Fonderie” of February 2008. In its introduction, this article sketches a review of the various families of alloys currently used and their relationship with forces undergone and architectures of modern cylinder heads.
It presents the recent developments in the field of alloys:
Alloy AlSi7Mg0.3, with the addition of 0.50% of copper and in state T7, a solution today used widely in industry, provides a very noticeable gain (+20%) of yield strength 250° C., without loss of elongation. But the gain provided by this small addition of copper is completely lost at 300° C.
The addition of 0.15% of zirconium in the same alloy makes it possible to slightly improve the yield strength at 300° C. (+10%) and especially to delay tertiary creep at the same temperature at a stress of 22 MPa.
A new type of AlSi7Cu3.5MnVZrTi alloy without magnesium was examined and characterized. It has excellent hot mechanical resistance properties at 300° C. and fairly good ductility throughout the field 20-300° C., but low yield strength at ambient temperature (about 190 to 235 MPa depending on its exact copper content). This alloy is in conformity with patents FR 2 857 378 and EP 1 651 787 by the applicant.
The results of these latest developments are summarized in table 1 below (tensile strength Rm in MPa, yield strength Rp0.2 in MPa and elongation at break A as a percentage, representing the stress in MPa leading to a deformation of 0.1% after being held at the same temperature for 100 h):
TABLE 120° C.250° C.300° C.AlloyStateRp0.2RmARp0.2RmARp0.2RmAAlSi7Mg0.3Ti (Fe 0.15, Primary)T621129515.757692940-4541533222AlSi7Mg0.3Ti (Fe 0.15, Primary)T72572999.9556134.538.8404334.521.7AlSi7Cu0.5Mg0.3Ti (Fe 0.15, Primary)T72753279.8667334.539.5404434.621.8AlSi5Cu3Mg0.3 (Fe 0.7, Secondary)F1722372.11071335.85360861226AlSi7Cu3Mg0.3 (Fe 0.44, Secondary)T52092821.8701101740658.5AlSi5Cu3Mg0.25Ti (Fe 0.15, Primary)T73113582.592111166047623026AlSi7Cu3.3MnVZrTi (without Mg, Primary)T71953358.09512419667526AlSi7Mg0.3Ti (Fe 0.15, Primary)T72343686.01021331963772631.8
More recent research carried out by the applicant, and not published up to now, has shown that the low cycle fatigue strength (high stresses and, consequently, small number of cycles) of this type of alloy without magnesium was definitely lower than that of the AlSi7Cu0.5Mg0.3 alloy, which is a major handicap owing to the fact that cylinder heads undergo alternating forces at very high stresses close to the yield strength, in particular because of thermal cycling related to how the engines work.
The Wöhler curves in FIGS. 1, 2 and 3 represent the fatigue strength in tension (with a fracture probability of successively 5% shown as a light line on the left, 50% as a dark line in the middle and 95% as a light line on the right) according to the number of cycles.
It definitely appears that the number of cycles to failure, for stress levels of about 250 MPa, is limited to approximately 1000 to 2000 cycles for new alloys without magnesium (FIGS. 2 and 3), whether the copper level is 3.3% or 3.8%, against at least 20,000 for the AlSi7Cu0.5Mg0.3 alloy (FIG. 1).
In high cycle fatigue, under a lower stress, about 150 MPa, the strength of the two families becomes similar, and the research published in the article of the review “Hommes et Fonderie” of February 2008 showed that the stress limits at 10 million cycles on shell test specimens were even higher for the AlSi7Cu3.5MnVZrTi alloys without magnesium, or between 123 and 138 MPa against 115 MPa for the AlSi7Cu0.5Mg0.3 alloy.